It can be easy to spot this kind of thing. I recall a student paper that was very well written, although almost every paragraph contained a sentence or two of terribly garbled English. This was a foreign student with weak English skills, so it was pretty clear that the majority of the paper had been written or at least edited by someone else who was a skilled writer.

This was not a case of copying something from a source, because I avoid asking the same question twice, or asking a question for which a websearch can turn up anything useful.